Memphis coach Lionel Hollins knew that Courtney Lee was out for the Celtics, and that their lone true defender in the backcourt was Avery Bradley.

So Hollins inserted Jerryd Bayless at shooting guard, realizing Jason Terry could not defend him one-on-one and that Jordan Crawford’s strength never has been defense. Bayless scored a season-high 30 points, many of those coming when he got the ball after the Celtics’ guards had helped inside.

But Hollins may have exposed a defensive weakness in the Celtics. They need Lee more than they thought because he is capable of defending shooting guards. Bradley is most skilled as an on-ball defender and when Hollins realized that he was assigned to point guard Mike Conley, he kept Conley in the game so Bradley would have to defend him, leaving Bayless to scorch anyone in front of him.

What it exemplified is the Celtics’ lack of concentration on defense. The night before in Dallas, Brandan Wright scored a season-high 23 points. The dog days can mean poor execution and thus difficult losses.

The Celtics haven’t been the same since they had a 96-83 lead last Monday night against the Heat. They were outscored, 22-7, to end the game, the Heat winning, 105-103, as LeBron James took over and Jeff Green never scored again after a strangely long four-minute rest in the fourth quarter.

The Celtics blew Wednesday night’s game at New Orleans, going scoreless for the final 1:54 and relaxing as the Hornets continued to play hard. Coach Doc Rivers claimed the next morning that his team has a short memory, that the players have endured so many peaks and valleys during the Big Three Era that individual losses can’t possibly penetrate their psyche.

Then came Friday night’s defeat in Dallas, and after Saturday night’s loss, Rivers admitted that that was what had happened.

“We didn’t play great; I wish we would have played better,” the coach said about the 0-3 road trip. ‘The New Orleans game set up the trip, in my opinion. We gave that away. It’s amazing how spirit goes to me. If you win that game, I don’t know if you win either one of the next two, but you’re in a better mind-set. When you lose a game like that, it makes you feel bad and sometimes you play that way.”

So the two days off before the Knicks come to Boston with bad intentions Tuesday night are critical. The Celtics should know how long Lee and Kevin Garnett will miss with their sprained left ankles. And they will have a much-needed practice Monday to work on some of their defensive issues.

Green, who has averaged 11.7 points since his 43-point outburst against the Heat, said the club hasn’t played entirely together, and that’s obvious. There is a lack of communication on defense. There is also the sense that the players believe matters will work themselves out, because the Celtics aren’t playing with the same sustained passion as they did a few weeks ago.

Against New Orleans and Memphis, they got off to strong starts then fizzled when the opponent offered resistance. Against Dallas, they fell behind, 11-4, and when they found enough fortitude to make a push, they would slip again, allowing Wright or the ageless Vince Carter to score a big basket or grab a key rebound.

The Celtics played with so much unrelenting confidence for so long during this stretch without Rajon Rondo (they’re now 16-10 after starting 14-4) that it’s surprising that it has dissipated so quickly. They must regain that swagger.

The next step is preparing for a first-round series against most likely the Knicks or Pacers, both winnable sets with the right execution and mind-set. Rivers took the important step last week of telling team president of basketball operations Danny Ainge that he wanted to keep the roster intact, so they retained Shavlik Randolph and D.J. White, and both were instrumental in the Celtics’ late surge at Memphis.

So while that last rally provided confidence — despite coming in a defeat — the frontline players now have to return to form for the Celtics to be a legitimate contender in the Eastern Conference. And let’s face it; they are probably the only team the Heat probably would want to avoid if possible.

Rivers said after Saturday night’s game that the Celtics finally played with the spirit he wanted in their 33-point fourth quarter. But for this club to actually fulfill its lofty expectations, the players have to show the passion they displayed when the NBA world doubted them.